The present invention relates to an apparatus for measuring a difference in level at a minutely rugged or stepped portion formed on a sample (more particularly, a level difference on the surface of a semiconductor device), by using an optical technique in a broad sense including not only visible light but also infrared and ultraviolet rays (more particularly by using an optical interference system).
Semiconductor devices are now being changed into three-dimensional configurations from two-dimensional configurations to achieve high integration. Under the circumstances, a technique is required for forming a hole with its depth about 3 to 5 .mu.m or a groove with its width about 1 to 2 .mu.m in a silicon substrate by suitable processes such as etching. For the development of such processes and the management of processing, a non-contact method for measuring the depth (level difference) of a processed groove is necessary. However, with the conventional technique, it was difficult to achieve the non-contact level difference measurement.
One analogous technique is a method utilizing an optical interference method as a means for measuring the depth of a rugged pattern of such as a video disc (see JP-A-54-17872). In this method, the pattern depth is measured in non-contact manner while changing the wavelength of an incident light. However, since this method is directed to the measurement of an average depth of a three-dimensional pattern, it is not suitable for measurement of the depth of a single hole positioned at a specific region (in the order of 0.2 to 3000 .mu.m.sup.2). The reason is because an optical system for illuminating only such a single hole or an optical system for detecting light from only the single hole is not provided.
As an apparatus having an image-forming or imaging optical system utilizing interference and diffraction of light, there is known a phase-contrast microscope whose principle is disclosed in various documents (e.g. Hiroshi Kubota, "Wave Optics", pp. 418-422 published by Iwanami Shoten, and H. P. Kleinknecht et al, "Optical Monitoring of the Etching of SiO.sub.2 and Si.sub.3 N.sub.4 on Si by the Use of Grating Test Patterns", J. Electrochem. Soc., Solid-State Science and Technology, Vol. 125, No. 5, pp. 798-803, May 1973).
The phase-contrast microscope can only observe clear contours of objects with phase difference by utilization of the phase difference but cannot quantatively obtain the phase difference or level difference between the objects. In other words, it cannot measure the depth of hole or groove.